1. Music today typically has more involved themes than "Let It Snow" and being Holly Jolly. It tends to be more complicated and, let's face it, agressive. I just can't see "Have a Hip Hop, Gangsta Rap Christmas" album coming out any time soon.
2. Overall, the younger generations are moving away from religion in general and Christianity (Christmas = Christ's Mass) in particular. I think the critical thinking skills and the ability to access many, many alternate points of view via the Internet has a lot to do with that. Couple that with the first point, and why would an atheist care enough about Christmas to create a song to celebrate something he or she doesn't believe in?
3. Money: Who owns the majority of radio stations in this country? Baby Boomers. Who dictates what gets played? The owners of the stations.
4. Fear: Our leaders are successfully working to maintain elevated fear levels in the population so they can point to that as a reason to keep them in power. When was the fear level at an equal level? The communist threat of the 50s and 60s. The music at the time soothed the fearfulness with simplistic themes and hopes of a magical being taking all the trouble away. Makes sense to me that with the return of amorphous fears (war on terrorism, islamism, war on drugs, war on [fill in noun here]), the music would be once again pointed to simplistic and hopey themes.
Just my thoughts. For what they're worth.